Ion intercalation in layered moo3 and WO3 nanostructure

Figure 3.2 a SEM image of grown muscovite mica substrate with KxMoO3 nanobundles extending out of substrate and MoO3 microbelts lying down on substrate b Typical morphology of individual

Ion Intercalation in Layered MoO3 and WO3

Nanostructure

Hu Zhibin

NATIONAL UNIVERSITY OF SINGAPORE

2013

Ion Intercalation in Layered MoO3 and WO3 Nanostructure

Hu Zhibin

(B Sc.)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN SCIENCE

DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE

2013

Acknowledgements

I would like to express my sincere gratitude to my supervisor Assoc Prof Sow

Chorng Haur I am greatly indebted to his inspirational motivation, selfless guidance

and immense support during the course of my Ph.D I am extremely thankful to him for

providing thoughtful suggestions and continuous hard work on papers

I would like to thank Assoc Prof Cheng Hansong for his guidance and support I

am grateful to him for providing theoretical calculations which are important to my

project I would like to thank Assoc Prof Tok Eng Soon for helping with theory

explanation of photoelectrical response effect

I would like to express my sincere thanks to Dr Varghese Binni, Dr Lim Zhihan,

Dr Hoi Siew Kit, Dr Wei Dacheng, Dr Zhou Chenggang for successful collaboration

I owe a deep sense of gratitude to all my group members Zheng Minrui, Mukherjee

Bablu, Yun Tao, Tamang Rajesh, Lim Xiaodai Sharon, Lu Junpeng, Ramanujam Prabhakar Rajiv for their support and I would also like to thank Wang Yinhui, Ji Zhuang for their collaboration I would like to thank Ms Foo Eng Tin for assisting with

lab suppliers as well

I acknowledge National University of Singapore (NUS) for research scholarship

I feel a deep sense of gratitude to my parent for their inspiration and affection

shown to me I am equally thankful to my fiancée Tang Wei for her understanding and

tolerance and for her simple presence by my side

Table of Contents

Declaration i

Acknowledgements ii

Table of Contents iii

Summary vi

List of Publications viii

List of Tables ix

List of Figures x

List of Symbols xvi

Chapter 1 Introduction 1

1.1 Wide applications of MoO3 and WO3 1

1.2 Intercalation induced new properties and intercalation method 6

1.3 Nanostructure induced new properties and nanomaterial synthesis method 9

1.4 Challenge of intercalating large ions into nanostructure 11

1.5 Research Aims 14

1.6 Outline of the thesis 15

Chapter 2 Experimental Techniques 16

2.1 Fabrication of Mo and W oxide nanostructures 16

2.2 Characterization Methods and Techniques 17

2.3 Individual Nanostructure Electrode Device Fabrication 20

2.4 Focused Laser System 22

Chapter 3 Intercalate K ions into MoO 3 layered nanostructure 24

3.1 Synthesis of K ion intercalated MoO3 nanobundle 24

3.2 Characterization of K ion intercalated MoO3 nanobundle 25

3.3 Theoretical Simulation of KxMoO3 nanobundle structure 38

3.4 Growth Mechanism of KxMoO3 nanobundle 40

3.5 Summary of Results 44

Chapter 4 Electrical Conductivity and Photo-Electrical Response 46

4.1 Electrical Measurement 46

4.2 Band Structure Analysis 52

4.3 Photoelectrical Response Measurement 54

4.4 Photon induced Electrical Response Measurement 57

4.5 Photon enhanced Electrical Response Measurement 65

4.6 Summary of Results 68

Chapter 5 Electromigration of K ions between MoO 3 layers 70

5.1 Introduction 70

5.2 Electromigration of K ions Detected by EDX 72

5.3 Structural Characters for Electromigration 77

5.4 Remnant Voltage induced by Accumulated K ions 78

5.6 Time dependence of Electromigration 84

5.7 Reversible Electromigration observation 86

5.8 Summary of Results 90

Chapter 6 Synthesis and Characters of K enriched WO 3 nanostructure 91

6.1 Introduction 91

6.2 Synthesis of K ion intercalated WO3 93

6.3 Characterization of K ion intercalated WO3 Nanobundle 96

6.4 Electrical Properties of KxWO3 Nanobundle 104

6.5 Theoretical Simulation of Lattice Structure and Band Structure 108

6.6 Photoelectrical Response Measurement 111

6.7 Comparison with KxMoO3 nanobundle 117

6.8 Summary of Results 120

Chapter 7 Conclusion and Future Works 122

Bibliography 131

Summary MoO3 and WO3 have been widely studied for their broad applications in many industry fields, including photochromic devices, electrochromic devices, ion batteries, gas sensors and catalysts The properties of these two materials can be significantly improved by either intercalation or nano-configuration It is thus reasonable to intercalate ions into nanostructured MoO3 and WO3 to achieve better properties for the two materials However, existing methods, which combine intercalation and nano-configuration, have various limitations, such as structure deformation upon ion intercalation, multi-step process and ion size limitation

This dissertation describes a simple one-step method to synthesize MoO3 and

WO3 single crystalline nanostructure with a great amount of K ion intercalation These two materials (KxMoO3, KxWO3 nanobundles) are fabricated by thermal evaporation

on mica substrate Despite the large amount of K ion intercalated (K:Mo/W>0.2), the layered and orthorhombic structure of MoO3 and pseudo-orthorhombic structure of

WO3 are preserved The method is simple and straightforward It utilizes the open ended furnace only and is carried out in ambient and moderate temperature The simpleness makes the method repeatable in other environment

Upon significant amount of ion insertion, many new properties are observed in MoO3 and WO3 nanostructures, including high conductivity, photoelectrical response and electromigration behaviour Firstly, the electronic conductivity of MoO3 or WO3

is enhanced by 7 orders in the case of MoO3 and 5 orders in the case of WO3 after ion insertion The magnitude is also three orders higher than that of the lithiated MoO3 bulk and five orders higher than that of lithiated MoO3 nanobelt The conductivity is further increased hundreds of times, when the material is heated from room temperature to

power (2.2 mW) without external bias voltage Remarkably, the amplitude and polarity

of the voltage can be controlled by the location of focused laser spot Finally, due to the large current density and the preserved layered structure, when an electric current is applied to a KxMoO3 nanobundle, the K ions migrate readily and rapidly in the flowing direction of electrons within the nanobundle

The simple preparation method provides a new direction to insert great amount of large ions into nanostructured materials without changing the structure of the materials The charge transferred from inserted ions results in extremely high conductivity, modifies the band structure of the material, and induces photon-electron response Moreover, the high current density, the single crystalline structure and the great amount of inserted ions will bring many unexpected phenomena into semiconductor nanostructures, such as electromigration behaviour It is noted that

KxMoO3 nanobundle and KxWO3 nanobundle are quite different with other materials which has the similar stoichiometry (such as potassium molybdenum bronze) Our materials are quite new and not reported before Besides the excellent properties described in the thesis, lots of properties are not systematically studied Future work are required to explore the materials and get further insight about the synthesis method

List of Publications

1 Zhibin Hu, B Rajini Kanth, Rajesh Tamang, Binni Varghese, Chorng-Haur Sow

and P K Mukhopadhyay, Visible microactuation of a ferromagnetic shape memory

alloy by focused laser beam, Smart Mater Struct. 21, 032003 (2012).

2 Zhibin Hu, Chenggang Zhou, Minrui Zheng, Junpeng Lu, Binni Varghese,

Hansong Cheng and Chorng-Haur Sow, K-enriched MoO3 nanobundles: a layered

structure with high electric conductivity, J phys Chem C, 116, 3962-2967 (2012)

3 Zhibin Hu, Zhuan Ji, Wilson Weicheng Lim, Bablu Mukherjee, Chenggang Zhou,

Eng Soon Tok and Chorng-Haur Sow, K-enriched WO3 Nanobundles: High Electric

Conductivity and Significant Photocurrent with Controlled Polarity, ACS Appl Mater

6 Bablu Mukherjee, Zhibin Hu, Minrui Zheng, Yongqing Cai, Yuan Ping Feng, Eng

Soon Tok, and Chorng Haur Sow, Stepped-surfaced GeSe2 Nanobelts with High-gain Photoconductivity, J Mater Chem 22, 24882 (2012)

7 Siew-Kit Hoi, Zhibin Hu , Yuan-Jun Yan, Chorng-Haur Sow and Andrew A

Bettiol A microfluidic device with integrated optics for microparticle switching, Appl

8 Dacheng Wei, Lanfei Xie, Kian Keat Lee, Zhibin Hu, Wei Chen, Chorng Haur

Sow, Yunqi Liu, Hongjie Dai, Andrew Thye Shen Wee, Controllable unzipping for

List of Tables

Table 3.1 The measured and calculated lattice constants of the MoO3 microbelt and the KxMoO3 nanobundle

Table 3.2 Atomic percentage of compounds in mica

Table 6.1 The measured lattice constants of WO3 powder and KxWO3 nanobundle

List of Figures

Figure 1.1 Periodic Table with blue rectangle denoting the transition metals

Figure 1.2 (a) unit cell of MoO6/WO6 octahedra (b) structure of orthorhombic MoO3

(c) structure of monoclinic WO3

Figure 2.1 (a) Schematic representation of the synthesis system (b) Photograph of the

tube furnace used to fabricate nanomaterial

Figure 2.2 Schematic images display the fabrication process of individual nanobundle

electrode

Figure 2.3 Optical image of (a) Laser Writing system (b) Sputtering system

Figure 2.4 Schematic of photon response measurement set-up with focused laser beam

radiation on nanostructure

Figure 3.1 (a) Schematic representation of the synthesis system (b) Photograph of the

tube furnace used to fabricate nanomaterial

Figure 3.2 (a) SEM image of grown muscovite mica substrate with KxMoO3

nanobundles extending out of substrate and MoO3 microbelts lying down on substrate (b) Typical morphology of individual MoO3 microbelt (c) Zoom in image in the middle where nanobundles grow highlighted by black square in (a) (d) Typical morphology of

a single KxMoO3 nanobundle

Figure 3.3 (a~b) zoom in image of the end of KxMoO3 nanobundle (c) Nanobelts split from each other in the left end of KxMoO3 nanobundle (d) KxMoO3 nanobundle broke

in the middle, inset image is the enlarged broken edge of nanobundle highlighted by black square

Figure 3.4 EDX spectrum of MoO3 microbelt (upper curve) and KxMoO3 nanobundle (lower curve)

Figure 3.5 XPS spectrum of MoO3 microbelt (upper curve) and KxMoO3 nanobundle (lower curve)

Figure 3.6 Electron diffraction pattern of the MoO3 microbelt on the (010) surface The highlighted yellow rectangle denotes the orthorhombic lattice structure The inset image shows a SEM image of the typical MoO3 microbelt growing in the [001] direction

Figure 3.7 (a~c) Electron diffraction pattern K MoO nanobundle on (010) surface,

KxMoO3 nanobundle growing in [001] direction (d) HRTEM image of KxMoO3

nanobundle, inset image is FFT analysis of select area highlighted by white square

Figure 3.8 (a~b) Black square spots shows calculated lattice constant a, c in different

nanobundles with varied atomic percentage ratio of K over Mo, red lines are the fitted lines correspondingly

Figure 3.9 The XRD spectrum of the mica substrate A with the MoO3 microbelts (upper chart) and the mica substrate B with both the MoO3 microbelts and the KxMoO3

nanobundles (lower chart) The label peaks with M are muscovite peaks while the label peaks without notation are MoO3 peaks The three peaks that are labelled with asterisks denote the layered structure of KxMoO3 correspond to expand along (020), (040) and (060) The rest of the peaks could be attributed to other faces of KxMoO3

Figure 3.10 Raman spectrums of individual MoO3 microbelt and KxMoO3

nanobundle on Si substrate

Figure 3.11 The optimized structure of (a) the pure MoO3, and KxMoO3 structure with (b) K as intercalants, (c) K as occupants and (d) mixed In the structures, red balls represent O atoms, blue balls represent Mo atoms and purple balls represent intercalated K atoms In the mixed case, the purple and green balls represent intercalants and occupants, respectively

Figure 3.12 (a) Optical microscope image of flat mica surface after growth for 20 min,

inset image is the liquid island after 30 min growth (b) Cracked mica surface after 20 min growth (c-e) schematic of growth process of KxMoO3 nanobundle

Figure 4.1 (a) SEM image of individual nanobundle electrode device (b) Schematic

image of the side view of device for electrical measurement (c) Zoom in image of the gap between electrodes (d) Side view of electrodes

Figure 4.2 I-V curves of KxMoO3 nanobundle and MoO3 microbelt

Figure 4.3 I-V curves of KxMoO3 nanobundle at different temperatures Inset image shows the set up

Figure 4.4 Temperature dependence conductivity of the nanobundle in log scale at

voltage of 4 V

Figure 4.5 (a) Schematic setup of FET device (b) Current (Isd) versus source drain voltage (Vsd) curves recorded at different gate voltages (0, +10 V and +40 V) for the device shown in (a)

Figure 4.6 Calculated Band structure and the Density of States (DOS) of KxMoO3

Nanobundle

Figure 4.7 Simulated Lattice Structure of KxMoO3 nanobundle, inset image display the flow path of electrons, where reduced Mo atoms align Red balls represent O atoms, blue balls represent Mo atoms and purple ones represent K atoms

Figure 4.8 SEM image of the KxMoO3 nanobundle contacted with metal electrodes Inset shows zoom in image of middle segment of nanobundle between two electrodes

Figure 4.9 Schematic of photon response measurement set-up with focused laser beam

radiation on KxMoO3 nanobundle

Figure 4.10 Schematic of focused laser beam locally irradiating at four different

locations and four optical images showing the position of laser spot on KxMoO3

nanobundle device

Figure 4.11 Laser spot is directed on location Ⅳ, photon induced voltage and current

are measured without externally applied bias voltage respectively in (a) and (b)

Figure 4.12 (a) Schematic of location dependence measurement process (b) Photon

induced voltage at different distance between center of laser spot and center of nanobundle

Figure 4.13 Photon induced current measured when laser spot is directed at four

different locations without externally applied bias voltage Inset schematic image displays the measurement set up

Figure 4.14 Normalized photon induced voltage at the moment laser is present at four

different locations

Figure 4.15 Photon induced voltage under different laser power when laser is directed

at location Ⅳ

Figure 4.16 (a) Schematic of focused laser beam locally irradiated at four different

locations under external bias (b) Photocurrent measured under external bias voltage of 0.4 V with laser spot directed at location Ⅲ on KxMoO3 nanobundle device

Figure 4.17 The Photon enhanced current at different distance between center of laser

spot and center of nanobundle

Figure 4.18 (a) Typical I-V characteristics of nanobundle with and without laser spot

shown at location Ⅲ respectively Inset curve shows the IV behaviour at low bias (b) Photon enhanced current under different external bias voltage

Figure 5.1 SEM image of a typical segment of nanobundle between two gold

electrodes

Figure 5.3 Measured atomic percentage ratio of K over Mo versus distance away from

electrode 1 in the sample before applied bias voltage (black square), after applied positive bias voltage (red square) and after applied negative bias voltage (blue square)

Figure 5.4 Log scale of EDX intensity measurement from the area highlighted by black

square in Figure 5.1 before (black curve) and after (red curve) application of electric current

Figure 5.5 (a) Structure of K enriched MoO3 nanostructure (b) Zoom-in image of area highlighted by black square in (a)

Figure 5.6 Schematic figure shows remnant voltage measurement process

Figure 5.7 After bias voltage (7 V, 5 V, 2 V and -2 V) is applied for 20 s, remnant

voltage between two electrodes is measured with time respectively

Figure 5.8 Schematic images of K ions dispersion process

Figure 5.9 (a) Measured starting remnant voltage after different bias voltage is

removed (b) Measured starting remnant voltage versus external bias current applied

Figure 5.10 After bias voltage 2 V is applied for 20 s, normalized remnant voltage

between two electrodes is measured versus time under different temperature

Figure 5.11 The exponential decay time of remnant voltage under different

temperature

Figure 5.12 After bias voltage (+3 V) is applied for 10 s and 100 s, red and black curve

show measured remnant voltage versus time respectively

Figure 5.13 After bias voltage (+3 V) is applied for 10 s, the remnant voltage versus

time is shown by red curve After bias voltage (-3 V) is applied for 100 s and then bias voltage (+3 V) is applied for 10 s, blue curve shows measured remnant voltage

Figure 5.14 Schematic image describes the reversible electromigration process Figure 5.15 Current increases with time when bias voltage (+2 V) is applied in sample

without bias voltage applied before (black line) and with bias voltage (-2 V) applied before (blue line)

Figure 5.16 Schematic image describes the distribution of K ions at the moment the

polarity of external bias voltage reverses

Figure 6.1 Schematic representation of (a) monoclinic WO3 in the [001] direction (b)

W5O14 in the [001] direction with a net-work of hexagonal and pentagonal columns

Figure 6.2 Schematic of the synthesis system of KxWO3

Figure 6.3 XRD spectrum of (004) surface of Mica substrate upon heating in different

temperature for 2 hrs

Figure 6.4 (a) Schematic of the position nanobundles grow (b) Top view SEM image

displays nanobundle grown on the cleavage of mica substrate

Figure 6.5 (a) Typical morphology of a single KxWO3 nanobundle The inset image is a zoom in image of the left end of the nanobundle (b) Zoom in image of the end of nanobundle

Figure 6.6 EDX spectrum of individual nanobundle on TEM grid

Figure 6.7 XPS spectrum of W4f peaks in nanobundle The raw data (red curve) is

fitted by W6+ peaks (blue dash curve) and W5+ peaks (black solid curve)

Figure 6.8 (a~b) Electron diffraction pattern of the KxWO3 nanobundle along [010] zone axis, the highlighted blue rectangle formed by large bright spots represents the lattice structure of the K intercalated WO3 The inset image shows a TEM image of the typical KxWO3 nanobundle growing in the [001] direction

Figure 6.9 Electron diffraction pattern of the nanobundle along the [122] zone axis

Figure 6.10 Lattice constant (a) a and (b) c at various atomic percentage ratio of K over

W

Figure 6.11 XRD spectrum of WO3 powder (upper curve) and KxWO3 nanobundles (lower curve)

Figure 6.12 SEM image of individual nanobundle contacted by electrodes

Figure 6.13 I-V curves of WO3 powder (black) and individual KxWO3 nanobundle (red)

Figure 6.14 (a) Schematic setup of FET device (b) Current (Isd) versus source drain voltage (Vsd) curves recorded at different gate voltages (0, +20 V) for the device shown

Figure 6.19 Schematic of focused laser beam locally irradiating at three different

locations and three optical images showing the position of laser spot on nanobundle device

Figure 6.20 Photon induced current measured when laser spot is directed at location

Ⅰ without externally applied bias voltage

Figure 6.21 Photon induced current at different distance between center of laser spot

and center of nanobundle at zero bias Two red broken lines denote the edge of electrode and the nanobundle lies between two lines

Figure 6.22 Photocurrent measured under external bias voltage of 4 V with laser spot

directed at location Ⅱ on the nanobundle device Inset shows schematic of focused laser beam locally irradiating at three different locations under external bias

Figure 6.23 The Photon enhanced current at different distance between center of laser

spot and center of nanobundle under external bias voltage of 4 V

Figure 6.24 The morphology of mica substrate after the growth of (a) KxMoO3

nanobundle (b) KxWO3 nanobundle

Figure 6.25 The lattice constant a under different atomic percentage ratio of (a) K

over Mo in KxMoO3 nanobundle (b) K over W in KxWO3 nanobundle

Figure 7.1 The optical image of nanobundle (a) before applied voltage (b) after applied

voltage and current flow from electrode 1 to electrode 2 for ~1min (c) after applied current for ~3 min (d) ~5 min (e) ~7 min (f) ~9 min (g) ~10 min The schematic image below each figure shows the color of KxMoO3 nanobundle at each step

Figure 7.2 SEM image of individual KxMoO3 nanobundle (a) before annealing (b) after annealing at 450 for 20 min (c) zoom in image of the edge of nanobundle in (b) (d) The nanobundle fully transforms into new KxMoO3 nanostructures

Figure 7.3 (a) Electron diffraction pattern of KxMoO3 nanobundle on (010) surface, the yellow rectangle constructed by large bright spots represents lattice structure of K intercalated MoO3, inset image shows TEM image of typical KxMoO3 nanobundle

growing in [001] direction, white arrows highlight the superstructure (b) Electron

diffraction pattern of the KxWO3 nanobundle on (010) surface, the blue rectangle formed by large bright spots represents the lattice structure of the K intercalated WO3 The inset image shows a TEM image of the typical KxWO3 nanobundle growing in the [001] direction White arrows highlight the superstructure

τ Life time of charge carrier

L Charge carrier diffusion length

Chapter 1 Introduction

In daily life, transition metal oxides are widely used, such as natural magnets (Fe3O4), pigments in all colors used in plastics, glass, ink, ceramics, paints and coating (CuO, Ti-Ni-O, ZnO), sunscreen and UV light absorber in cosmetics and skin care products (TiO2) With such a great variety of applications, transition metal oxides constitute one of the most interesting classes of solids1 Transition metal oxides are materials containing transition metals and oxygen, while transition metals are metallic elements that serve as a transition between two sides in periodic table as shown in Figure 1.1 Among these transition metal oxides, MoO3 and WO3 have been widely studied due to their interesting layered structures Because of their unique structures and properties, MoO3 and WO3 have a wide range of applications in many different industry fields, including photochromic devices2, electrochromic devices3, ion batteries4 , gas sensors5 and catalysts6

Figure 1.1 Periodic Table with blue rectangle denoting the transition metals.7

MoO3 and WO3 have a variety of structures including orthorhombic structure, monoclinic structure, hexagonal structure and tetragonal structure Among these various MoO3 and WO3 structures, orthorhombic MoO3 and monoclinic WO3 are two

of the most common structures in MoO3 and WO3 family Both materials are built up

by MoO6/WO6 octahedra Each octahedra (Figure 1.2(a)) contains a central Mo/W atom surrounded by six oxygen atoms with almost the same distance from the central atom In the orthorhombic MoO3 structure, corner-sharing MoO6 octahedra are linked

as a chain, and two similar chains are connected together by edge-sharing to form layers of MoO3 (Figure 1.2(b))2 These layers are stacked in a staggered configuration and held together by weak van de Waal´s forces In the monoclinic WO3 structure, infinite array of corner-sharing WO6 octahedra forms a layer of WO3, and these layers are stacked in arrangement and held together by weak van de Waal´s forces (Figure 1.2(c)).8

Figure 1.2 (a) unit cell of MoO6/WO6 octahedra (b) structure of orthorhombic MoO3

(c) structure of monoclinic WO3

In both structures, tunnels (continuous vacancies) are formed between layers in which small ions could stay or move in the event of the presence of an exterior force The ability to accommodate ion insertion in this kind of structure makes it an

and ion batteries Moreover, both materials are easily reduced, a property which enables it to be applied as catalyst Besides, the MoO3 and WO3 are n-type semiconductors, the electric property of which is sensitive to the exposed environment, making them good candidates for gas sensors.5 The detailed behaviours and mechanisms of these applications will be discussed below

Application 1: Photochromic devices

Photochromism is the reversible transformation of a species between two forms with different colors upon the absorption of photo radiation The reversible photochromism effect is observed in the MoO3 films Specifically, the films are almost transparent in the visible region which turns blue under UV-light irradiation The colored film can be bleached by electrochemical polarization, and be colored again

by UV irradiation.2, 9 In particular, the change of colors corresponds to the difference

in light absorption The transparent MoO3 films exhibit strong absorption of light with wavelength below 400 nm, and the absorption arises from electron excitation from valence band to conduction band For the colored film, significant absorption appears

in the visible light range with the maximum absorption wavelength between 780 nm and 900 nm, which results from a superposition of electron excitation to many discrete bands.10

Many theoretical models try to explain the photochromism effect, including color center model, double insertion/extraction of ions and electrons model and small polaron model2 Double insertion/extraction of ions and electrons model is mostly accepted The model indicates that when irradiated with UV light, electrons and holes are formed in MoO3 film, while holes react with adsorbed water (the prepared film always contain water on the surface or inside the structure) to generate protons (H+).2,

10 These protons diffuse into the MoO3 lattice and react with photon generated electrons and MoO3 to form hydrogen molybdenum bronze (HxMoO3) In the bronze, part of Mo atoms are reduced to Mo5+, and due to the injected photon energy from

light, intervalence charge transfer occurs from the newly formed Mo5+ to adjacent

Mo6+ The absorption of light in certain wavelength makes the film turn blue

The photon induced color change phenomenon has made MoO3 and WO3 as promising candidates for many technological applications, such as large scale display, erasable optical storage media, radiation intensity controller, self-developing photography and so on.2

In the simplified model,3, 11 upon ion and electron insertion diffused from electrolyte, the following reaction takes place

WO3+ xM++ xe− ↔ MxWO3with M+ = H+, Li+, Na+ or K+ In tungsten bronze MxWO3, part of W6+ ions are reduced to W5+ The inserted electrons are localized in these W5+ sites and polarize their surrounding lattice to form small polarons These small polarons absorb the energy from incident photons and hop from one site to another, as in

hν + W5+(A) + W6+(B) → W6+(A) + W5+(B) where hν is the energy of absorbed photon energy and polaron hops from A to B Due

to absorption of photons with certain wavelength, the film turns to dark blue

The electrochromic effect has rendered MoO3 and WO3 as promising candidates for many technological applications12 such as “smart” window,13, 14 Electrochromic

Application 3: Ion battery

Lithium-ion batteries are quite popular these days In lithium ion battery, the positive electrode is mostly made of Lithium cobalt oxide, contributing Li ions The negative electrode is made of materials with layered structure, where Li ions could be inserted and extracted When the battery charges, Li+ moves through the electrolyte from the positive electrode to the negative electrode and inserts between layers in electrode During discharge, the lithium ions move back to the LiCoO2 from the cathode.4, 17

As described above, in MoO3 and WO3, continuous vacancies are formed between layers in which small ions could stay or move in the presence of exterior force For the ability of reversible incorporation of Li ions into MoO3 or WO3 layered structure, they are used as the cathodes in lithium batteries.4, 18

Application 4: Gas sensor

A reducing molecule (e.g., CO, H2) adsorbed on the sensor surface injects electrons into it While oxidizing gases like NO, extract electrons from sensor surface Consequently, in the case of n-type semiconductors, the resistance of the sensor decreases when contacts with reducing gas due to high electron population, while the resistance increases when contacts with an oxidizing gas.5 The changed resistance upon appearance of gases contributes to the sensitivity of MoO3 and WO3 to these

gases

Nitrogen oxides, NO and NO2, produced from combustion facilities and automobiles are main pollutants which damage human respiratory organs and nerves

WO3 have outstanding sensitive properties toward NOx at low and elevated

temperature Tong et al.19 reported that WO3 thin film sensors exhibit high sensitivity

to NO2, and the lowest detection concentration of NO2 is 1 ppm Meanwhile, the sensitivity to 10 ppm NO2 gas is five times larger than the response to 500 ppm CO,

H2S, CH4 gases Compared with other materials, the low cross sensitivity makes WO3

better candidate for detection of these two gases

Ammonia is a reducing agent for nitrogen oxides converting them into nitrogen and water vapor An ammonia sensor is required to selectively detect small quantities

of ammonia fed into the inlet stream in the presence of interfering gases such as NOx,

CO and hydrocarbons.20 MoO3 is sensitive to NH3 and NO2 gases in the temperature range from 200 to 450 oC, particularly the lowest detection concentration of NH3 is 3 ppm No cross sensitivity was recorded for other gaseous species.20, 21 The particular selectivity makes MoO3 a better candidate for NH3 based integrated sensors

continuously be partially oxidized

1.2 Intercalation induced new properties and intercalation method

As described above, in the layered structure of MoO3 and WO3, tunnels form

their applications as photochromic devices, electrochromic devices and ion batteries

In addition, intercalating ions into the structure of MoO3 and WO3 improves the performance of these devices, such as displaying higher coloration efficiency, faster response, stronger absorption, higher stability and so on

Photochromic devices: Yao et al.23 reported that upon Li intercalation into MoO3, the sample showed a new photosensitive response in the region between 500 and 800

nm, while the pure MoO3 was sensitive only to the UV light (wavelength below 400 nm) This is attributed to the fact that upon Li+ incorporation, the shortest Mo–Mo separation distance increases and results in the formation of a new energy state

Therefore, electrons can be excited into a new state after irradiation with visible light, and then to the conduction band of molybdenum oxide The process reduces Mo6+ to

Mo5+, resulting in the formation of blue-colored molybdenum bronze

Electrochromic device: Zhang et al.24 reported that Li intercalation into MoO3

film dramatically improved the electrochemical reversibility of the film In the Li-doped MoO3 films, some degradation was observed between the third and the tenth cycle but a pronounced improvement in durability was achieved In contrast, pure MoO3 film showed significant irreversible insertion in the first cycle with subsequent degradation in the second cycle

Batteries: Leroux et al.25 reported that upon sodium (Na) intercalation, MoO3

film exhibited high specific capacity of 940 mA/g in the voltage window of 3.0~0.005V, and the charge capacity efficiency (the ratio of the charge capacity at nthcircle over the value at first circle) was 88% after the 20th cycle and 75% at the 50thcycle, while in pure MoO3, the capacity at first cycle was around 700 mA/g and the capacity efficiency decreased to 67% at the 10th cycle The comparison indicates that upon Na intercalation, higher charge capacity and more stable charge capacity retention is achieved

Gas sensor: Wang et al.19 intercalated Polyaniline (PANI, a kind of polymer) into the MoO3 layered structure The resulting (PANI)xMoO3 thin film displayed an increase in electrical resistivity in response to volatile organic compounds The intercalated film was especially sensitive to formaldehyde and acetaldehyde, while pure MoO3 was not sensitive to either of the gases These two volatile gases are poisonous to humans Formaldehyde is highly toxic to all animals, regardless of method of intake Ingestion of as little as 30 mL of a solution containing 37% formaldehyde has been reported to cause death in an adult human Acetaldehyde, air pollutant resulting from combustion like automotive exhaust and tobacco smoke, is toxic when applied externally for prolonged periods, which possibly induce cancer

Besides the reports mentioned above, many species could be intercalated, thus improving the performance of MoO3 These species include ions like H+, Li+, Na+, K+,

Rb+, Cs+, Ce3+26 and many kinds of polymers like PANI, SP, pyridines27 There are many methods to intercalate these species into MoO3/WO3 structure These methods could be divided into two groups, electrochemical method and self diffusion method

Electrochemical method: Spahr et al.28 and Sian et al.29 carried out insertion experiments in galvanostatic mode using a standard three-electrode arrangement, in which the working and counter electrodes were soaked with an electrolyte solution Poly-MoO3 films were used as a working electrode, platinum strip as a counter electrode, Ag/AgCl as a reference electrode, and an appropriate quantity of HClO4, LiClO4, NaClO4, KClO4, Mg(ClO4)2 dissolved in propylene carbonate (PC) as electrolyte solution for insertion of H+, Li+, Na+, K+ and Mg2+ ions separately Driven

by external electric field, these ions were intercalated between MoO3 layers

Self diffusion method: Tagaya et al.27 intercalated organic compound into MoO3

by two steps Firstly, MoO3 was suspended in an aqueous solution of Na2SO4 to form

ethanolic solution containing guest organic compound Zhang et al.30 dissolved metallic molybdenum powder in H2O2 solution and then added LiOH•H2O By drying the solution at 40℃ in an oven, Li-doped MoO3 xerogel powder can be produced

Mahajan et al.31 fused niobium pentoxide (Nb2O5) powder with potassium pyrosulphate (K2S2O7) in silica crucible The solution was dissolved in tartaric acid and mixed with MoO3 powder dissolved ammonia solution The final solution was pneumatically pulverized on the glass, and the Nb-doped MoO3 film was achieved after spraying During these processes, compounds were intercalated by self diffusion method driven by concentration gradient force

It is noted that these methods are applied to MoO3 films or powders, and these materials turn into an amorphous structure after a great amount of ion intercalation or large size ion intercalation

nanomaterial synthesis method

The materials in nano-scale are widely studied these days and they exhibit various advantages, such as the high surface to volume ratio of the nanostructures provides large contact surface areas for reaction, the short diffusion path in the crystalline structure improves ion reaction efficiency, and the high flexibility and adequate toughness accommodate strains induced by ion insertion, etc The performance of MoO3 and WO3 in all application fields are greatly enhanced when both materials in nano-scale are utilized

Electrochromic device: The diffusion coefficient and the length of diffusion path determine the electrochromic efficiency The former depends on the crystal structure, and the latter is determined by the material’s microstructure.32 With same crystal structure, the materials in nanoscale display short diffusion length for its small size

and thus exhibit excellent electrochromic efficiency Se-Hee Lee et al.33 reported that

by fabricating Electrochromic films from crystalline WO3 nanoparticles, the cycling stability and electrochromic efficiency of EC device were dramatically increased In nanoparticle films, the current response increases slightly during the course of 3000 cycles without a significant change in the shape of the CVs, while the amorphousWO3film degrades significantly after only 500 cycles It indicates the excellent cycling stability of the nanoparticle films Moreover, the total cathodic charge for the WO3

nanoparticles is ~32 mCcm–2mg–1, compared to ~3 mCcm–2mg–1 for crystalline films and ~9 mCcm–2mg–1 for amorphous films The high charge insertion density over the same time period indicates great electrochromic efficiency of the nanoparticle films

Batteries: There are several advantages associated with the utilization of nanomaterial as electrode for lithium batteries, including (a) better accommodation of the strain induced by Li+ insertion/extraction, improving cycle life (b) new reactions not available with bulk materials (c) larger electrode/electrolyte contact area, leading

to higher charge/discharge rates (d) short electron transport path lengths, permitting operation under low electronic conductivity or at higher power (e) short Li+ transport path lengths, allowing operation under low Li+ conductivity or higher power.34Molybdenum oxide microrods with diameters of ~2–6 μm were investigated as a cathode and compared with ball-milled MoO3 particles.18, 35 The microrods maintained

a reversible capacity of 199 mAh/g which was 88.4% of the highest capacity, while the particle electrode exhibited a capacity of 85 mAh/g after 100 cycles corresponding to 47.2% of the initial capacity It indicates that the nanostructures could more easily

accommodate the structural strain occurred upon Li ion insertion Liang Zhou et al.36

reported that the α-MoO3 nanobelts exhibited a high discharge capacity of 264 mAh/g

at 30 mA/g and 176 mAh/g at 5000 mA/g The capacity was still up to 114 mAh/g after 50 cycles at the high current density of 5000 mA/g, while the bulk α-MoO3

cracked during the same process The excellent high rate performance is related to the

Sensor: For the large surface to volume ratio of nanomaterials, their conductivity

is strongly influenced by surface reactions, which significantly improves their sensitivity to gases.37 E Comini et al.38 reported that on the response to CO, MoO3

nanorods showed the response to 30 ppm of CO was more than 100% while the thin film showed the response of 30% only Moreover, the pure single crystalline of nanorod guaranteed long term stability of the sensor, which was not always satisfied in bulk due to grain coalescence

Large scale synthesis of MoO3 and WO3 nanostructure is achievable using various techniques ranging from direct thermal heating of foil in a furnace, hot wire

chemical vapour deposition, hydrothermal, sol-gel methods, and etc Chu et al.39 and

Xie et al.40 reported that by heating the Molybdenum foil to ~500-600°C in a tube furnace or on hot plate, the oxidized vapour nucleated and nanostructure formed on

the substrate covered on or nearby the foil Zhou et al.36 synthesized MoO3 nanobelts

by hydrothermal treatment of a peroxomolybdic acid solution, which was prepared by mixing H2O2 aqueous solution with Mo metal powder Dillon et al.32 employed hot-wire chemical vapor deposition (HWCVD) to produce MoO3/WO3 nanostructure

A single W/Mo filament, was resistively heated to ~1400 °C in a static gas atmosphere consisting of Ar and O2 gases, and the metal oxide powder collected on the walls of the quartz tube as the filament was slowly oxidized

Considering that MoO3 and WO3 can achieve better performance through intercalation and nano-configuration, it is reasonable to intercalate ions into nanostructure so that better properties of MoO3 and WO3 are obtained However, existing methods, which combine intercalation and nano-configuration, have various limitations

One of the methods which intercalate ions into a MoO3 thin film is the electrochemical method Intercalated species invariably take the interstitial positions between layers of the MoO3 structure With the uptake of these ions, the interlayer spacing of the MoO3 increases The over expansion induced from large size ion intercalation or from a great amount of small size ion intercalation destroys the layered structure of MoO3 The phenomenon has been observed in experiments

carried out by many groups Sian et al.41 reported that upon intercalation of 20% K ions (large size ion, atomic percentage ratio of K over Mo is 0.2), the layered structure

of MoO3 is destroyed due to the over expansion between layers The structural change phenomenon is illustrated by the disappearance of (020) peak in XRD measurement after intercalation, in which the (020) peak denotes the layered structure of MoO3

Joseph et al.42 reported that upon immersing MoO3 film into dilute (~10-9 M) LiClO4

solution for a long time (60 min, intercalation of small ions Li+ with a great amount), the layered structure of MoO3 deforms step by step over the course: firstly cracks associated with the formation of LixMoO3 become prominent at the (010) surface of MoO3, then the cracks extend along other directions, and finally, the structural integrity of the (010) surface is diminished, the film disintegrates and flakes off

In addition to the limitation of structural deformation, the electrochemical method is also constrained in the application into nano-configuration During the process of intercalation into nanostructures by the electrochemical method, a metallic substrate is required as electrode in the electrochemical process to hold the MoO3

nanostructures (nanobelts, nanowires, nanoparticles) Zhou et al.36 pressed the powder mixture of MoO3 nanobelts, acetylene black and PTFE into thin film, and the film was

placed onto Ni grid for the electrochemical process Meduri et al.35 directly synthesized MoO3 nanowires onto stainless steel substrate The conducting substrates, together with as synthesized nanowires, were used as electrodes for the electrochemical process In these processes, multi-steps are required to achieve ion

intercalation in nanostructure; moreover, structural deformation in nanostructure still exists after ion intercalation

Self diffusion method is also a widely used method to intercalate ions into nanostructures For small ions (such as Li+), lithiated MoO3 nanobelts were prepared

by immersing MoO3 nanobelts into LiCl solution43 However, the efforts to intercalate large ions such as K+ into the MoO3 nanostructure by self diffusion method has never been successful due to the large size of these ions compared to the size of the gap between layers

Besides physical ion insertion, there are some synthesis methods to intercalate ions by chemical reaction However, the structure of MoO3 and WO3 deforms greatly

by these methods Insertion of K+ ions in the synthesis of bulk potassium molybdenum bronze (K0.3MoO3) by electrolytic reduction of potassium molybdate and molybdenum oxide mixtures gives rise to a substantial structural distortion The compound becomes infinite sheets consisting of clusters of ten edge-sharing molybdenum octahedral linked

by corners in the [010] and [102] directions with the adjacent sheets held together by potassium ions.12 Zheng et al synthesized potassium tungsten bronze nanowires by

thermal heating potassium hydroxide solution treated tungsten foils.44 In the compound, WO6 octahedra are formed into a six member ring and K ion occupies the vacancy in the center of the ring

To date, intercalating large cationic species into MoO3/WO3 nanostructures without giving rise to severe structural deformation of the layered orthorhombic MoO3

structure or monoclinic WO3 structure has remained a great technical challenge

1.5 Research Aims

In this study, we synthesize MoO3 and WO3 single crystalline nanostructure with

a great amount of K ion (large size ion) intercalation These two materials are fabricated by a simple method - thermal evaporation on mica substrate The muscovite mica acts as K source and substrate for growth With the simple one step method, we achieve single crystalline, nano-configuration and ion intercalation and preserved crystal structure at the same time, while in other reports, a multi-step method was used or structure became deformed Despite the large amount of K ion intercalated (atomic ratio of K over Mo/W is larger than 0.2), the layered and orthorhombic structure of MoO3 and pseudo-orthorhombic structure of WO3 are preserved The simple preparation method solves the problem that large size ion intercalation deforms the structure of MoO3 and WO3, and thus provides a new direction to develop nano-structured materials of large-ion intercalated metal oxides

A significant amount of ion insertion in the nanostructure induces many new properties, including in particular, high conductivity, photocurrent response and electromigration behaviour are observed in our materials Firstly, the electronic conductivity of MoO3 and WO3 is enhanced several orders upon K insertion, and the value is further increased hundreds of times, when the material is heated from room temperature to 200 degree The substantial high electric conductivity attributed to ion intercalation has not been observed in other reports Secondly, the significant photoelectrical response of individual nanomaterial (KxMoO3, KxWO3 nanobundle) is observed Before ion intercalation, both materials do not display such photocurrent response due to the large band gap of the materials The comparison suggests that the method presented in the thesis provides an excellent way to introduce photoelectrical response property to the material Finally, the rapid and reversible electromigration of intercalated K ions within a layered single crystalline K MoO nanobundle is observed,

The electromigration is always observed in the metal and induces cracks or piles in the material, but the phenomenon has not been observed in semiconductors before The observation in our material opens a new route for the study of electromigration effect and provides new insight about such effect

1.6 Outline of the thesis

The structure of the thesis will be as follows In the Chapter one, we have introduced the properties and applications of MoO3 and WO3 and the enhanced performance induced by ion intercalation and nanostructure configuration The synthesis methods to intercalate ions and to produce nanostructure are introduced as well The challenge of intercalating large ions into nanostructure is discussed and the aim of the thesis is proposed In Chapter two, we will introduce the experimental techniques used in the project including synthesis method, characterization techniques, individual nanostructure electrode device fabrication method and focused laser system

In Chapter three, the synthesis method to intercalate K ions into the single crystalline MoO3 nanostructure and the characterization of the material will be discussed in detail Chapter four introduces the electrical properties of K enriched MoO3 nanobundle and the observation of photocurrent In Chapter five, we look into the amazing phenomenon of electromigration of intercalated K ions between MoO3 layers In Chapter six, we apply similar method to successfully synthesis K enriched WO3

nanobundle, further characterization and applications are introduced Finally, we conclude our work and discuss the future works in Chapter seven

Chapter 2 Experimental Techniques

In this chapter, we describe the thermal evaporation method to synthesize K ion intercalated MoO3 and WO3 nanostructure, and the various experimental characterization techniques utilized to identify the structure of the material The preparation process of individual nanostructure electrode device for electrical property measurement is provided The alignment of focused laser system for individual nanostructure photo-electrical response measurement is discussed as well

2.1 Fabrication of Mo and W oxide nanostructures

Our group has successfully fabricated MoO3 nanobelt and WO3 nanowire using thermal evaporation method on glass and silicon substrate.40 Similar thermal evaporation method is utilized to fabricate K enriched MoO3 nanostructure A horizontal tube furnace (Carbolite MTF 12/25/250) is used for the controlled growth

of nanostructure by thermal evaporation method The tube furnace contains a ceramic tube of diameter ~ 6 cm with both ends open in the ambient We utilize a Mo/W foil (5 mm×5 mm×0.05 mm in size, from Aldrich Chemical Co., Inc.) as Mo/W source The foil is polished by sand paper to remove the surface oxidized layer Then, these foils are cleaned by sonicating in deionized water and isopropanol for 15 mins respectively, air dried in ambient and placed in the middle of ceramic boat A muscovite mica sheet (K2O•3Al2O3•6SiO2•2H2O, 8 mm×8 mm in size, from Alfa Aesar Co., Inc.) is prepared as substrate and K source The mica sheet is placed on top

of the Mo/W foil at certain height in the ceramic boat The ceramic boat is inserted into the tube furnace and placed in the middle, the hottest region of the tube furnace The

After growth, the system is cooled down to room temperature in ambient During the synthesis, air flow is controlled by fan to provide adequate oxygen continuously The synthetic scheme described above is shown in Figure 2.1(a) Figure 2.1(b) displays a photograph of the tube furnace used to fabricate nanomaterial

Figure 2.1 (a) Schematic representation of the synthesis system (b) Photograph of the

tube furnace used to fabricate nanomaterial

2.2 Characterization Methods and Techniques

As-synthesized products are characterized using various characterization tools These characterization methods include morphology characterization by SEM, elemental composition and chemical state detection by EDX and XPS, lattice structure determination by TEM, XRD and Raman Spectroscopy

Scanning Electron Microscope (SEM)

Scanning Electron Microscope (SEM) is the most widely used equipment to study surface features of materials in micro- and nano-scale A focused electron beam scans

on the samples and reacts with the surface atoms The scattering interaction between beam electrons and surface atoms results in the ejection of secondary electrons These secondary electrons are originated within a few nanometers from the sample, used for imaging.45 The morphological characterization of the grown substrate and the structures created in this work is carried out using the field emission SEM JEOL

JSM-6700F with the spatial resolution of ~10 nm

Transmission Electron Microscopy (TEM)

Transmission Electron Microscopy (TEM) is the technique with advantage of

large magnification range In TEM microscopy, a low magnification image of the nanostructures and a high resolution image of the alignment of atoms can be achieved The low resolution TEM images provide the information about the size, shape and morphology of the nanostructures The high resolution TEM images provide information about lattice structure, crystalline quality and details of the defect structures In TEM, images are formed by a beam of electrons transmitting through the specimen Because the wavelength of high-energy electrons is a few thousandths

of a nanometer while the spacing between atoms in the solid is hundreds times larger, the atoms act as a diffraction grating to the electrons When electrons transmit through the sample, some of these electrons are scattered to particular angles, determined by the crystal structure of the sample The image on the screen will be a series of spots,

named Selected Area Electron Diffraction pattern (SAED), telling the crystal

structure of the material.46 The TEM analysis is carried out using JEOL JEM-2010F

with 200 kV electron beam

Energy-dispersive X-ray spectroscopy (EDX)

Energy-dispersive X-ray spectroscopy (EDX) is an analytical technique used for the elemental analysis A high energy beam of electrons is focused on the sample, excites an inner shell electron in sample atoms, ejecting it from shell and creating

a hole An electron in the outer shell fills the hole, and releases the energy equalling to the difference between the higher-energy shell and the lower energy shell in the form of X-ray The detected energy of the X-rays is characteristic of the atomic structure of the element, allowing the elemental composition of the specimen to be determined.47 The EDX measurement system is equipped in SEM or TEM

X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is an analytical technique that measures the elemental composition and chemical state of the elements in the material A X-ray beam is irradiated towards the material, and the top 1 to 10 nm of the sample absorb the X-ray energy, ejecting outer shell electrons The kinetic energy of these electrons are measured and the elements in the material are determined.48 The XPS spectrum is sensitive to the chemical environment of an atom It enables the application of XPS in the identification of valance state of a particular element The XPS spectrum is recorded under ultra-high vacuum condition using Omicron EA125 analyzer; monochromatic Mg Kα source (1253.6 eV), system

X-ray Diffraction (XRD)

The regular aligned atoms in the lattice structure scatter the incident wave and result in the diffraction The significant diffraction appears when the wavelength of the incident wave is comparable with the distance between atoms For the wavelength

of X-ray is the same order of the magnitude as the spacing between planes in the crystal, X-ray Diffraction (XRD) spectrum is utilized to reveal the information about the crystal structure of materials by detecting the scattered intensity of an X-ray beam The X-ray beam is originated from the source, and directed onto the sample The scattered X-ray beam is recorded by the detector as a function of incident and scattered angle We record the XRD spectrum using Philips X’PERT MPD (Cu Kα (1.5418 Å) radiation) machine

Raman Spectroscopy

Raman Spectroscopy characterizes the material by analysing vibrational, rotational and other low frequency modes in a sample A focused laser beam is directed onto the sample and a spectrometer is utilized to detect the intensity of inelastic scattering of photons after photon-phonon interaction within the material Results are recorded in terms of the difference in wavenumbers between the incident and scattered

photon, known as the Raman shift The Raman spectra reported in this works are taken using Renishaw Ramascope 2000 system with an Olympus microscope attachment and

a 514.5 nm laser used as the excitation source

2.3 Individual Nanostructure Electrode Device Fabrication

To investigate the electrical properties of individual nanostructure, the micro-electrode device is fabricated Individual nanostructure is transferred from the growth substrate to SiO2/Si substrate and photolithography method is utilized to achieve designed metal (Au(400 nm)/Cr(10 nm)) finger electrodes (of gap ~15 μm) that make contact with the nanostructure Photolithography is a process used

in micro-fabrication to make patterns on the thin film Focused laser beam is used to transfer a pattern onto a light sensitive photoresist on the substrate A series of chemical treatments are then applied to engrave the exposure pattern and enable deposition of other materials upon the pattern The remaining photoresist is then removed and leaving the patterned materials on the substrate

Figure 2.2 Schematic images display the fabrication process of individual nanobundle

electrode

UV lithography method is utilized to fabricate individual nanobundle electrode device 7 schematic images in Figure 2.2 illustrate the preparation process The Si wafer with 100 nm SiO2 layer is properly cut into 1 cm x 1 cm size These SiO2/Si substrates are cleaned by sonicating in deionized water and isopropanol for 15 mins respectively, and dried by nitrogen gas flow (step 1) Individual KxMoO3 nanobundle is then transferred to the centre of clean substrate by the needle of micro-probe-station under microscope (step 2) The positive photoresist AZ1518 is spin coated onto the substrate with the thickness of 2 µm (step 3) The thickness of photoresist is optimized, while thinner one could not properly cover the nanobundle and thicker one will result

in inconvenience of removing polymer We design electrode patterns by the help of AutoCAD software, and load the design into the computer that controls the laser writing system The laser writing system we use is uPG101 from Heidelberg Instrument

Co as shown in Figure 2.3(a) The substrate is placed in the centre of platform There are holes on the platform and air pumping is performed, which tightly suck substrate on the platform by air pressure UV laser is focused onto the substrate and moving around controlled by computer (step 4) The laser power is optimized according to the thickness of photoresist The written substrate is then immersed into a developer, removing the part exposed to UV laser (step 5) The patterned substrates are collected and placed inside the sputtering chamber The sputtering system is displayed in Figure 2.3(b) In the system, plasma hit the metal target, driving atoms ejected from target Most of the metal atoms diffuse to nearby substrates, and finally, a layer of material is covered on the patterned substrate In our case, 20 nm Cr layer is first sputtered on the substrate to firmly adhere the Au layer with SiO2 substrate 500 nm Au layer is then deposited on the Cr layer (step 6) The sputtered substrate is treated by acetone solution, removing the remaining photoresist and the metal layer covering on it (step 7) In the end, the device with Au finger electrodes covering on individual nanobundle is fabricated

Figure 2.3 Optical image of (a) Laser Writing system (b) Sputtering system

After the fabrication of electrode device, copper wires are used to connect the device with sourcemeter to form a circuit One end of copper wires are connected to the electrode by soldering and the other end of wires are connected to crocodile clips The electrical measurements are carried out using Keithley 6430 Sub-fA Remote SourceMeter

2.4 Focused Laser System

To investigate the photo-electrical response of individual nanostructure, laser beam should be focused and directed onto nanostructure Thus, the focused laser system is set up to couple laser beam into microscope Here we will describe in detail the components of the focused laser system